A solid polymer-type fuel cell (hereinafter referred to as a “fuel cell” in some cases) is a power-generating device that generates electricity using the chemical reaction of a fuel gas (examples thereof include hydrogen) with oxygen, and is greatly expected to play a role as one of the next-generation resources for energy in the fields of electric appliance industry, automobile industry, or the like. The fuel cell is composed of a basic unit having two catalyst layers and a polymer electrolyte membrane interposed between the two catalyst layers.
When the power-generating mechanism of a fuel cell using hydrogen as a fuel gas as a typical fuel cell is briefly described, hydrogen is ionized on one catalyst layer to produce hydrogen ions, and the produced hydrogen ions are conducted (ion conduction) to the other catalyst layer through the polymer electrolyte membrane, in which the hydrogen ions are reacted with oxygen to form water. In this case, when the two catalyst layers are connected to an external circuit, an electric current flows to supply electric power to the external circuit. The ionic conduction of the polymer electrolyte membrane is exhibited by the movement of ions along with the movement of water through hydrophilic channels in the polymer electrolyte membrane, and thus, it has been required to maintain the polymer electrolyte membrane in the wet state in order to exhibit ionic conduction efficiently. Such a power-generating mechanism makes the wet state of the polymer electrolyte membrane constituting the fuel cell changed according to the starting and stopping of the fuel cells. When the wet state of the polymer electrolyte membrane is changed as described above, the polymer electrolyte membrane is subjected to alternating between swelling and shrinkage by water absorption/drying, and thus, a defect sometimes occurs in that the interface between the polymer electrolyte membrane and the catalyst layer is microscopically damaged. Further, in extreme cases, failure of the fuel cell may also be caused.
Therefore, the polymer electrolyte membrane used in fuel cells is required to be capable of exhibiting ionic conductivity efficiently at a low water absorption ratio enough to further reduce the swelling and shrinkage (dimensional change from water absorption) according to water absorption and drying.